Electroluminescent from cooled,homo-geneous gallium sulfide crystal



May'fi, 1969 E. MOOSER ET AL 3,443,141 ELECTROLUMINESCENT FROM COOLED, HOMOGENEOUS GALLIUM SULFIDE CRYSTAL Filed Aug. 4, 1966 COLLE 70R 6 v.......... INVENTORS.

7 E UEL MOOSE 84$ 4 MAN I? Y JOHN LOW BREBNER rromvsr United States Patent US. Cl. 313-108 5 Claims ABSTRACT OF THE DISCLOSURE The invention relates to devices and processes in which an extraordinary property of gallium sulfide is employed advantageously. The devices and processes utilize the newly discovered property of gallium sulfide, namely that electroluminescence once started in the gallium sulfide will continue under conditions less extreme than those required for the electroluminescence.

This invention relates to devices and processes utilizing electroluminescence of gallium sulfide and in a more particular aspect the invention includes electroluminescence which is photon induced.

Many substances are capable of electroluminescence under the influence of voltage alone, and some substances will electroluminesce if subjected to irradiation with photons of suitable energies and in suitable amounts at voltages at which electroluminescence would not be initiated by voltage alone in the dark. With the electroluminescent materials which have been used hitherto, electroluminescence ceases if the conditions which initiate electroluminescence are reduced. For many purposes it would be desirable for the electroluminescence to continue when the conditions for initiation are changed, either by reducing the voltage or eliminating the external irradiation in the case of photon induced electroluminescence. Such an action, which is somewhat analogous to a thyratron tube, which will maintain conduction at voltages below that required for firing, or some gaseous discharge tubes, such as fluorescent lighting tubes, has not, however, been practical in the case of electroluminescent materials which have been available hitherto.

The present invention depends on an extraordinary property of gallium sulfide which has been discovered, namely that electroluminescence, once started, will continue under conditions less extreme than those required for initiating the electroluminescence. The invention may be considered both from the standpoint of devices which utilize the newly discovered property of gallium sulfide or, in another aspect, of processes for such utilization. Both aspects are included in the present invention.

Gallium sulfide under suitable conditions, preferably at low temperatures, such as the boiling point of liquid nitrogen or the boiling points of other elemental gases which boil at low temperatures, exhibits the new characteristics briefly referred to above. For example, if the gallium sulfide is submitted to a sufficiently high potential in the dark, electroluminescence starts, and then the potential may be reduced very substantially and electroluminescence will continue until the voltage is reduced to a much lower value, which will be referred to in the remainder of the specification as threshold voltage, abbreviated V It is another property of gallium sulfide that at voltages above V but below the voltages for initiating electroluminescence in the dark, which will be designated in the specification as V electroluminescence can be initiated by suitable irradiation of sufiicient intensity and with sufiiciently energetic photons. This includes ultra 3,443,141 Patented May 6, 1969 violet, and especially visible and shorter wavelength infrared radiation. Once electroluminescence is initiated at the voltage between V, and V electroluminescence continues even though the actuating irradiation is stopped. This is quite ditferent from the behavior of other materials. For example, if zinc sulfide activated with manganese and chlorine is subjected to voltage slightly below the voltage at which it electroluminesces in the dark and'is then irradiated with ultraviolet radiation, electroluminescence starts, but when the irradiation is stopped the electroluminescence ceases. These investigations were notably described in the book on electroluminescence by Henisch, pages 275-277, and in a letter to the editor by Dr. Cusano in the Phys. Rev., volume 106 (1957), pages 604606. Thin layers or flakes of the above referred to compounds are sometimes called Cusano cells. The cell can be used for radiation amplification and the same holds true for gallium sulfide crystals of the present invention. However, the newly discovered characteristic of gallium sulfide referred to above permits maintenance of electroluminescence even after irradiation is stopped. This" permits devices and processes for performing functions which cannot be performed with the Cusano cell or with other electroluminescent material which requires a higher voltage to initiate electroluminescence. It has not been determined exactly what the mechanism underlying the present invention is and therefore the invention is not intended to be limited to any particular theory of action.

A factor which is not fully understood at present is the behavior of gallium sulfide cells with different electrode contacts. Common evaporated electrode contacts, such as evaporated gold, zinc and the like, do not permit satisfactory operation. On the other hand, silver paste or paint, finely divided graphite, solders of indium or suitable low melting gallium related alloys such as those of indium, thallium, gallium, mercury, and the like.

A single layer or flake of gallium sulfide forms a satisfactory cell, or a stack of layers with intermediate conductors'may be used. Of course if the cell is to be used for photon triggering, contacts and conductors between layers must permit photons of the irradiation to strike the gallium sulfide. For example, the contacts may be transparent or partially transparent or they may cover only portions of the gallium sulfide, such as contacts in the form of a grid or other pattern. When the gallium sulfide is to be used for the effect in which conduction and electroluminescence is started by a higher voltage which then drops to a value between V, and V the requirements of transparency of electrodes or of transparent areas are not essential. In such a case, the electroluminescence of the gallium sulfide continues to permit operationsat the lower voltages regardless of whether any radiation from the exterior strikes the gallium sulfide. It should be noted that in the modification of the invention in which photon actuation is used to start electroluminescence at a voltage between V, and V it is not necessary that the radiation strike the gallium sulfide at a point in contact with an electrode contact. For example, a stack of gallium sulfide crystals can be irradiated from the side or edge, and once electroluminescence has started it continues.

The present invention is useful for a number of purposes, such as optical switching, in which a pulse of radiation of suitable intensity and photon energy starts electroluminescence and conduction continues until the voltage is reduced below V Thyratron like operations can be effected by applying pulses of higher voltage, for example, above V with steady maintenance of voltages between V, and V A gallium sulfide cell of the present invention in such a use would then conduct until the voltages drop below V which could be effected by a suitable pulse of the opposite polarity. Optical amplification can also be effected because the electroluminescence can be considerably more intense than a triggering radiation. A transistor-like configuration is also possible, which permits voltage amplification but not power amplification. Two important applications are image retention and image conversion. For example, an ultraviolet image can be converted to visible and the visible image can be retained even if the ultraviolet image is transient. Other uses will be apparent to those skilled in the art, the ones enumerated above being merely typical examples.

The invention will be described in greater detail in conjunction with the drawings, in which:

FIG. 1 is a curve of voltage vs. current, with a compressed ordinate scale;

FIG. 2 is a diagrammatic illustration of potential drops across a nonelectroluminescing gallium sulfide cell;

FIG. 3 is a similar diagrammatic representation of voltage drops under electroluminescing conditions, and

FIG. 4 is a diagrammatic illustration of a cell with three electrodes, analogous to a transistor.

Turning to FIG. 1, the abscissa represents applied voltage and the ordinate current, the voltages for V, and V appearing on the curve and the dark current curve beyond V, is shown in dashed lines, with a solid line curve showing the corresponding currents after electroluminescence has been started. At any voltage lying between V, and V electroluminescence can be photon actuated; V, is an illustration of such a voltage. The dashed lines may be considered as a metastable branch of the curves. The numbers representing voltages, namely about 20 volts fol-V and 30 volts for V are only illustrative as the exact voltages depend on the nature of the gallium sulfide cell and other factors.

It will be apparent from FIG. 1 that in the dark if the applied voltage is gradually increased, the current increases slowly along the metastable (dashed) branch until a voltage V is reached. Then electroluminescence starts, and the current increases to a very high value; thereafter, a relatively high current can be maintained with continuing electroluminescence at voltages below V as is illustrated by the solid curve. The gallium sulfide cell is useful in this mode as a thyratron-like device. When desired a protective series resistor may be used. Once electroluminescence has started, it will continue until and unless the voltage drops below V This shut-off can be effected in various ways, for example by a pulse of opposite polarity. The present invention is directed to processes and devices utilizing the newly discovered characteristics of gallium sulfide and is not intended to be limited to any particular circuit characteristics in which the devices or the process of the present invention is used.

If photon actuation is desired, the gallium sulfide cell may be subjected to voltage anywhere between V; and V and in the dark no electroluminescence will start. Then suitable irradiation, such as a flash of light or continued light, starts electroluminescence, and this will continue regardless of whether the external illumination continues and will also continue until the voltage is dropped below V FIG. 2 shows the potential drop across a gallium sulfide cell under conditions in which electroluminescence is not occurring. The two side areas, 1 and 2, are areas at the contacts, enormously exaggerated in width for clarity. The gallium sulfide itself appears at 3. It will be apparent that there is a small drop of potential at each contact and a relatively larger drop through the gallium sulfide itself. Applied voltage is shwon symbolically by the signs and on the drawing.

FIG. 3 shows the same cell with the same parts bearing the same reference numerals after electroluminescence has been started, either by a sufiiciently high voltage or voltage pulse or by photon actuation from suitable irradiation. Now the conductivity of the gallium sulfide itself becomes high, and therefore there is a relatively smaller drop of potential across the gallium sulfide itself, most of the potential drops occurring in the contact areas, Total current of course is very much greater, as can be determined from a consideration of the curves in FIG. 1. FIGS. 2 and 3 are idealized and exaggerated diagrammatic representations to make the electrical conditions more clear and are not intended to represent any particular voltage or current scale.

FIG. 4 is a diagrammatic showing of a gallium sulfide cell in the form of a thin layer, greatly exaggerated for clarity, with a contact configuration analogous to that of a transistor. The gallium sulfide layer is shown at 3 using the same numeral as in the preceding figures. One electrode 4, which is analogous to a base in a transistor, is shown with two other electrodes 5 and 6 analogous to emitter and collector in a transistor. The configuration resembles somewhat that of a field effect transistor. The voltage supplies for emitter and collector and the currents are represented by V V I and 1 respectively, in usual transistor nomenclature. The voltage V is always greater than V and, in the case of an operation analogous to that of a transistor, V is less than the threshold voltage represented by V on FIG. 1. For operation analogous to a thyratron, V must lie between V and V Considering FIG. 4 when operating as a transistor-like device, the electrodes may be of silver paste or paint and the applied emitter voltage V between emitter and base is large enough to make the crystal electroluminesce, or maintain the condition, once started. In other words, it is greater than V or V as the case may be. When a voltage signal is applied between 4 and 5, the collector current I will be modulated by the electroluminescent light emitted from the region between emitter and base. There is no power gain, but there is a voltage gain; in other words, the transistor configuration of the present invention is useful for voltage amplification.

When the collector voltage V is between V and V in FIG. 1, a voltage pulse which brings V above V on FIG. 1 will result in igniting the area between the base and the emitter, and this will cause current to flow also from the collector. The collector current will then be of the order of about a few microamps. By comparison in the unlit condition, the collector current is of the order of 10* to 10- amps. The action is analogous to a thyratron tube, and I is only turned olf if V and V are brought temporarily below V The thyratron-like operation may be used for applications for which this type of device is suitable, such as memory cores, high impedance switches, and the like.

When a cell such as is shown in FIG. 4, or in fact any gallium sulfide cell according to the present invention is used, care should be taken to connect the cell into circuits such that excessive currents will not be maintained. The precautions required in the circuits are the same as with ordinary thyratron tubes, transistors and the like, and as such form no part of the present invention. Therefore, no circuits are shown, since the configurations are conventional.

We claim:

1. A process for obtaining electroluminescence in a homogeneous gallium sulfide crystal which comprises cooling said crystal to low temperatures such as the boiling point of liquid nitrogen or the boiling points of other elemental gases which boil at low temperatures, applying an electrical voltage across the gallium sulfide greater than that at which electroluminescence, once started continues, and initiating the electroluminescence by applying a voltage sufilcient to initiate the electroluminescence in the dark, the voltage source electrically contacting the gallium sulfide by means of a member selected from the group consisting of silver paste, silver paint, finely divided graphite, and solders of indium or suitable low melting gallium related alloys, said voltages being applied to the gallium sulfide either prior to or after said cooling of the gallium sulfide, said electroluminescence being obtained from said crystal in said cooled state.

2 A process according to claim 1 for producing an action analogous to that of a thyratron in which the condition initiating electroluminescence is a transient application of voltage sufiicient to initiate electroluminescence in the dark.

3. A process for obtaining electroluminescence in a homogeneous gallium sulfide crystal which comprises cooling the crystal to low temperatures such as the boiling point of liquid nitrogen or the boiling points of other elemental gases which boil at low temperatures, applying an electrical voltage across said gallium sulfide greater than that at which electroluminescence, once started continues, but less than that required to initiate electroluminescence in the dark, and initiating electroluminescence by irradiating said gallium sulfide with photons gt sufiicient energy to initiate electroluminescence at the voltage applied across the gallium sulfide, the voltage source electrically contacting said gallium sulfide by means of a member selected from the group consisting of silverpaste, silver paint, finely divided graphite, and solders of indium or suitable low-melting gallium related alloys, said voltage and said-irradiation being applied to the gallium sulfide either prior to or after said cooling of said gallium sulfide, whereby said electroluminescence is obtained from said crystal in said cooled state.

4. A process according to claim 3 for light amplification which comprises external irradiation with photons of sufiicient energy to initiate electrolurninescence, the initiated electroluminescence being greater than the external radiation.

5. A process of image retention according to claim 3 in which the external radiation is in the form of an image, and is stopped after electroluminescence has been initiated.

References Cited UNITED STATES PATENTS 2,814,004 11/1957 Goodman 317-237 3,254,267 5/1966 Sack. 3,265,532 8/1966 Mooser et a1 3l7237 3,267,294 8/1966 Dumke et a1. 3,278,814 10/1966 Rutz. 3,283,160 11/1966 Levitt et a1. 3,290,539 12/ 1966 Lamorte. 3,305,685 2/1967 Wang. 3,307,089 2/ 1967 Yamashita. 3,330,991 7/1967 Lavine et al.

OTHER REFERENCES Henisch: Electroluminescence, Macmillan Company, New York, 1962.

ROBERT SEGAL, Primary Examiner.

US. Cl. X.R. 317-237 553 UNI ED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3aw-l-3y 4 Dated Hal 6, 1969 Patent No Inventor 3 Emanuel Mooser and John Low Brebner It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the drawing, "Electroluminescent" should read Electroluminescence Column 1, line 2, "Electroluminescent" should read Eleotroluminescenoe Column 2, line 35, insert after "like" are quite satisfactory Column 3, line 65,

"shwon" should read shown smNEMN-s SEALED sen-m i Attest:

WILLIAM E- 80m, 33%- Edwfl'a u Commissioner of Patents Attesting Officer 

